Heat sink and its manufacturing method

ABSTRACT

A heat sink including a substrate of insulating ceramic and, superimposed on its upper surface, a diamond film for disposing an element thereon. The heat sink has excellent heat radiation characteristics and is stable during use. This heat sink is obtained by providing a substrate of ceramic containing aluminum nitride of high thermal conductivity as a principal component; forming on the substrate a bonding member layer, such as a silicon film which is capable of being bonded with the substrate and a diamond film; and forming a polycrystalline diamond film of high quality on the bonding member layer so that the polycrystalline diamond film is strongly bonded with the substrate via the bonding member layer interposed therebetween in accordance with, for example, the vapor phase method.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a heat sink and a process for producingthe same. More particularly, the present invention relates to a heatsink having such a laminate structure that a layer of diamond of highthermal conductivity is superimposed on a substrate of aluminum nitrideof high thermal conductivity, and further relates to a process forproducing the heat sink.

2. Description of the Prior Art

The processing capacity of electronic components is making strikingenhancement in accordance with the increase of information density.Thus, currently a large amount of heat is emitted from each electroniccomponent. Since maintaining constant temperature is desired forensuring the stable functioning of electronic components, variousimprovements for cooling them have been proposed. It is common practiceto, in practical use, mount electronic components tending to have hightemperature on a heat radiating member, namely, a material capable ofabsorbing heat, or a device including such a material for thermallyprotecting a constituent element or a system known as a heat sink.

As materials having been in practical use in heat sinks for long, therecan be mentioned metals and metal alloys of satisfactory thermalconductivity, such as Cu and Cu—W, and semiconductive or insulatingceramics of high thermal conductivity, such as SiC and AlN. However, asa result of integration of electronic components, the outgoing heat isincreasing to such an extent that the cooling with the use of a heatsink based on these materials only encounters a limit. Consequently, aheat sink material based on diamond exhibiting the highest thermalconductivity (about 2000 W/mK) among the existent materials, has beendeveloped as a new heat sink material for enhancing heat radiationcharacteristics. As such a heat sink which is now common, there can bementioned a substrate of copper and, brazed thereto, a single crystaldiamond in plate or film form, known as a submount. However, the heatradiation characteristics of this heat sink have not always beensatisfactory because the single crystal diamond is so expensive that aheat sink of large configuration cannot be employed and because thebrazing material becomes a resistor to heat conduction.

Therefore, forming of a polycrystalline diamond film on a substrateaccording to the vapor phase synthetic process has been tried. Forexample, Japanese Patent Laid-open Publication No. 5(1993)-13843proposed a heat radiating member consisting of a substrate having asurface for mounting a semiconductor element thereon and, covering themounting surface, a diamond layer synthesized by the vapor phaseprocess. This heat radiating member is for the purpose of suppressingthe deterioration of performance of semiconductor laser elements bygenerated heat, and obtained by directly forming a polycrystallinediamond layer of 10 to 500 μm thickness on a substrate in accordancewith the microwave plasma CVD (Chemical Vapor Deposition) process. Itwould be feasible to use this heat radiating member as a submount byminiaturizing the same.

In the above heat sink, the most suitable material of substrate on whicha polycrystalline diamond film is to be formed varies depending on theusage. Among various substrate materials, sintered aluminum nitride(AlN) is known as a material having not only insulating properties butalso high thermal conductivity. The AlN substrate is especially usefulwhen it is intended to form a circuit on a substrate. Specifically, whenit is intended to form a polycrystalline diamond layer only on an areaof substrate on which an element is to be mounted while forming acircuit on areas of substrate other than the area of substrate on whichan element is to be mounted, an insulating material such as a ceramic ispreferred as a substrate material from the viewpoint that additionalformation of an insulating film is not needed. Moreover, a material ofhigh thermal conductivity is desired for avoiding the drop of heatradiation efficiency, namely, maintaining high thermal conductivity withrespect to the whole body of substrate. Therefore, the use of sinteredaluminum nitride (AlN) as a substrate material having not onlyinsulating properties but also high thermal conductivity is to beconsidered.

In this connection, in the use of a metal of high thermal conductivityas a substrate, it is possible to obtain a substrate having not onlyinsulating properties but also high thermal conductivity by laminatingthe substrate with an insulating film of, for example, SiO₂ inaccordance with the vapor deposition method or the like. However, thisinsulating film often poses a problem with respect to the reliability involtage withstanding properties, etc.

It is anticipated that the above heat sink comprising a substrate ofsintered aluminum nitride (AlN) and, superimposed thereon, apolycrystalline diamond layer will have high availability.

However, it is difficult to directly form a polycrystalline diamondlayer of high quality on a substrate of sintered aluminum nitride (AlN)in accordance with the vapor phase synthetic process. From the practicalviewpoint, any heat sink comprising a polycrystalline diamond layer ofhigh thermal conductivity superimposed on a substrate of AlN is notknown. For example, although the above Japanese Patent Laid-openPublication No. 5(1993)-13843 describes that Cu, Cu—W alloy, Cu—Moalloy, Cu—W—Mo alloy, W, Mo, sintered SiC, sintered Si₃N₄, sintered AlNand the like can be used as substrate materials, the thermalconductivity of the polycrystalline diamond actually formed on asubstrate of sintered AlN is as extremely low as 300 W/m·K.

It is an object of the present invention to provide a method of forminga polycrystalline diamond layer of high quality on a ceramic substratecontaining aluminum nitride (AlN) as a principal component. It isanother object of the present invention to provide a heat sink havingsuch a fundamental structure that a polycrystalline diamond film isformed on the above substrate, and having excellent heat radiationcharacteristics.

SUMMARY OF THE INVENTION

The present inventor has found that a polycrystalline diamond film ofhigh quality exhibiting high thermal conductivity can be produced on aceramic substrate containing aluminum nitride as a principal componentby preliminarily forming thereon a layer of specified substanceexhibiting excellent adhesion to the substrate and a diamond film andthereafter forming a polycrystalline diamond film on the above layer inaccordance with the vapor phase method.

Therefore, according to one aspect of the present invention, there isprovided a heat sink comprising a ceramic substrate having at least oneplane containing aluminum nitride as a principal component, and adiamond film layer superimposed on the plane of the ceramic substrate,characterized in that the ceramic substrate and the diamond film layerare bonded together via a bonding member interposed therebetween.

In the above heat sink of the present invention, a ceramic substratecontaining, as a principal component, aluminum nitride which isexcellent in insulating characteristics and heat radiationcharacteristics is used as a substrate material. Accordingly, the heatabsorption performance and heat radiation performance of the heat sinkas a whole are excellent. Further, in case of the heat sink having sucha structure that the diamond film is formed so as not to cover theentire surface of the substrate, an electronic circuit made of a metalcan be printed on the substrate without the particular need to form aninsulating film.

In the heat sink of the present invention, it is preferred that thebonding member be constituted of at least one material selected from thegroup consisting of silicon, silicon carbide, tungsten, tungstencarbide, CuW, Cu—Mo alloy, Cu—Mo—W alloy, amorphous carbon, boronnitride, carbon nitride and titanium. In the use of this bonding member,cracking or other faults of the diamond film superimposed on the bondingmember can be avoided, the heat radiation performance is enhanced, andthe durability of the diamond film is prolonged.

This bonding member may be constituted of a crystalline substanceorientated on a specified crystal face. In particular, it is preferredthat the bonding member be constituted of a polycrystalline siliconpreferentially orientated for crystal face (111), crystal face (220) orcrystal face (400.) When the bonding member is constituted of acrystalline substance, the crystal grains of the polycrystalline diamondfilm formed on the bonding member would be enlarged and thecrystallinity thereof would be high, so that the diamond film would haveespecially high thermal conductivity.

According to necessity, the polycrystalline silicon may contain adopant. The addition of the dopant would enable enhancing theorientation of polycrystalline silicon and would also enable impartingelectrical conductivity to the silicon film.

In the heat sink of the present invention, preferably, the diamond filmlayer exhibits a thermal conductivity of 800 W/mK or greater. The heatsink of the present invention attains enhanced thermal conductivity ascompared with that of a heat sink wherein a diamond film layer isdirectly superimposed on a substrate.

According to another aspect of the present invention, there is provideda process for producing a heat sink, comprising the steps of providing aceramic substrate having at least one plane containing aluminum nitrideas a principal component; forming a bonding member layer on at least aportion of the plane of the ceramic substrate; and forming a diamondfilm on the bonding member layer.

In the above process of the present invention, peeling or cracking ofthe diamond film would not occur during the cooling step after thediamond film formation step wherein the substrate is maintained at 700to 1100° C. Thus, a heat sink of high quality according to the presentinvention can be stably produced in high efficiency in the process ofthe present invention. In particular, when a layer of crystallinesubstance orientated for a specified crystal face is formed as thebonding member layer, a diamond film of large crystal grains and highcrystallinity can be easily formed. Moreover, when a layer containingsilicon as a principal component is formed as the bonding member layer,a diamond nucleation would be promoted, thereby enabling formation of adiamond film in higher efficiency. Furthermore, in the process whereinthe bonding member layer is formed from a conductive substance, such asa polycrystalline silicon containing a dopant, and wherein, while adirect current voltage is applied to the bonding member layer, a thindiamond film is formed on the bonding member layer in accordance withmicrowave CVD or hot filament CVD method, it would be feasible toefficiently deposit an effective precursor for diamond film synthesis onthe substrate surface, thereby easily forming a diamond film of highcrystallinity, high orientation degree and high thermal conductivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of one representative form of heat sinkaccording to the present invention;

FIG. 2 is a schematic view of radiofrequency plasma CVD apparatus whichcan be appropriately used in the formation of a silicon film as abonding member layer in the production process of the present invention;and

FIG. 3 is a schematic view of microwave CVD apparatus which can beappropriately used in the formation of a diamond film in the productionprocess of the present invention

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The heat sink of the present invention is a laminate comprising aceramic substrate having at least one plane containing aluminum nitrideas a principal component, and, a diamond film layer superimposed on theplane of the ceramic substrate, characterized in that the ceramicsubstrate and the diamond film layer are bonded together via a bondingmember interposed therebetween. The terminology “heat sink” used hereinmeans a material capable of absorbing heat emitted by the aforementionedelements, or a device including such a material for thermally protectinga constituent element or a system. Thus, the terminology expresses aconcept comprehending what is known as a submount.

In practical use, various elements such as a semiconductor element, aresistor and a capacitor are mounted on the diamond film.

The heat sink of the present invention includes a ceramic substratecontaining, as a principal component, aluminum nitride which is amaterial exhibiting high thermal conductivity despite having insulatingproperties. Therefore, even in the formation of a laminate with thediamond film, the drop of total thermal conductivity can be suppressed.Further, when the diamond film is provided on not the entire surface ofsubstrate but part of the surface of substrate, a circuit for wiring canbe simply printed on the part where no diamond film is provided by thevapor deposition of a wiring material such as gold.

The above substrate for use in the present invention (hereinafter alsoreferred to simply as “AlN substrate”) is not particularly limited aslong as it is constituted of a ceramic containing aluminum nitride as aprincipal component and as long as its configuration is one having atleast one plane for formation of a diamond film on which an element ismounted. For example, the substrate may be a plate produced by adding asintering auxiliary to aluminum nitride powder, shaping the mixture, forexample, under pressure, and sintering thereof. Also, the substrate maybe one obtained by molding a polycrystalline aluminum nitride to a plateform.

The above diamond film may be constituted of a polycrystal or a singlecrystal, and may be constituted of natural diamond or synthetic diamond.From the viewpoint of cost and easiness of film formation, however, itis preferred that the diamond film be constituted of a polycrystallinediamond synthesized by the vapor phase method. The area, configuration,thickness, etc. of the diamond film are appropriately determineddepending on the mounted element, desired heat radiationcharacteristics, production time and production cost. Generally, thethicker the, diamond film, the greater the heat radiation. On the otherhand, the thinner the diamond film, the less the production time andcost. Therefore, taking a balance of these into account, it is preferredthat the thickness of the diamond film be in the range of 10 to 300 μm,especially 20 to 250 μm.

The higher the thermal conductivity of diamond film, the greater theadvantage. In the employment of the production process of the presentinvention which will be described in detail later, hence, it ispreferred to use a polycrystalline diamond film exhibiting a thermalconductivity of 800 W/mK or greater, especially 1000 W/mK or greater, inthe heat sink of the present invention. In this connection, the diamondfilm is so thin that the thermal conductivity thereof cannot be directlymeasured. Therefore, the thermal conductivity of diamond film refers toa value calculated, after measuring of the thermal conductivity of aheat sink as a whole, from the thermal conductivity and thickness valuesof known materials constituting the substrate and the bonding member andthe measured thermal conductivity of the heat sink as a whole.

The most characteristic feature of the heat sink according to thepresent invention resides in that the AlN substrate and the diamond filmare bonded together via a bonding member layer interposed therebetween,wherein the bonding member has adherence to both the AlN substrate andthe diamond film. The interposition of the bonding member layer enablesproducing a diamond film of high quality without the peeling or crackingof diamond thin film during the production process. Further, after theproduction process, it enables avoiding the breakage or peeling ofdiamond film by a heating/cooling cycle during the use thereof.

This bonding member layer is not particularly limited as long as it is alayer constituted of a material having desirable adherence to both theAlN substrate and the diamond film. However, from the viewpoint of highadhesive strength (or bonding strength) to both the AlN substrate andthe diamond film, it is preferred that the bonding member layer beconstituted of at least one material selected from the group consistingof silicon, silicon carbide, tungsten, tungsten carbide, CuW, Cu—Moalloy, Cu—Mo—W alloy, amorphous carbon, boron nitride, carbon nitrideand titanium.

When the bonding member is constituted of a crystalline substanceorientated for a specified crystal face, the crystal grains of thepolycrystalline diamond film formed on the top of the bonding memberwould be enlarged and the crystallinity thereof would be high, so thatthe thermal conductivity of the diamond film would be enhanced.Accordingly, it is preferred that the bonding member be constituted ofsuch a crystalline material. Such a crystalline material can be, forexample, a polycrystalline silicon preferentially orientated for crystalface (111), crystal face (220) or crystal face (400); or apolycrystalline silicon containing a dopant, such as boron orphosphorus, preferentially orientated for the above crystal face. Thedoped orientated polycrystalline silicon is especially preferred fromthe viewpoint that, because it is conductive, a direct current voltagecan be applied at the forming of diamond film as described later.

Although the thickness of the bonding member layer is not particularlylimited, it is preferred that the thickness be in the range of 5 nm to 3μm, especially 10 nm to 2 μm, from the viewpoint of a balance of bondingeffect, effect on easiness in obtaining a diamond film of high quality,time taken in the formation of bonding member layer, drop of thermalconductivity by providing of the bonding member layer, etc.

FIG. 1 is a representative sectional view of heat sink “A” according tothe present invention. The heat sink “A” has such a structure thataluminum nitride substrate 100 in plate form is laminated with bondingmember 110 and diamond film 120 in this order. Although FIG. 1 shows theform of heat sink wherein the entire upper surface of the aluminumnitride substrate 100 is covered by the bonding member 110 and thediamond film 120, it is not always needed for the bonding member layerto adhere to the entire lower surface of the diamond film as the upperlayer, and it is satisfactory that partial adherence be effected, aslong as satisfactory bonding strength can be realized.

The process for producing a heat sink according to the present inventionis not particularly limited. For example, the heat sink can beappropriately produced by forming the bonding member layer on at least aportion of the plane of the AlN substrate and forming the diamond filmon the bonding member layer so that the diamond film covers at least aportion of the bonding member layer. With respect to the bonding memberlayer formed in this production process, it is not particularly limitedas long as the formation is effected so that part or the entiretythereof is finally interposed between the AlN substrate and the diamondfilm. The formation of the bonding member layer may be effected in theconfiguration of a film having an area larger than that of the diamondfilm, or a film having an area smaller than that of the diamond film.Moreover, the film-like configuration is not always needed. For example,the bonding member layer may be in the form of a lattice, or a pluralityof spots scattered at given intervals therebetween. It is preferred thatthe thickness of the bonding member layer be in the range of 5 nm to 3μm from the viewpoint of a balance between attained effects andproductivity.

In the formation of the bonding member layer, suitable methods accordingto the material of the bonding member layer is appropriately selectedfrom among the common film forming methods known as being useful in theformation of films on substrates without limitation. Examples of suchfilm forming methods include printing, plating, vapor deposition,chemical vapor deposition (CVD), sputtering and laser abrasion methods.Of these, the vapor deposition and chemical vapor deposition methodsenable forming a film of high purity with high film thickness precision,thus providing especially effective means.

For example, the bonding member layer constituted of at least onematerial selected from the group consisting of silicon, silicon carbide,W, WC, CuW, Cu—Mo alloy, Cu—Mo—W alloy, titanium and BN can beappropriately formed by the vacuum vapor deposition method with the useof electron beams. In this method, a material consisting of the sametype of substance as that constituting the bonding member layer isplaced on a hearth in a vacuum chamber. This material is exposed toelectron beams, so that the material is melted and vaporized. The vaporof the material is deposited (vapor deposited) on a surface of substratedisposed in the vacuum chamber. Thus, the formation of the bondingmember layer is accomplished. At this stage, the thickness of depositedfilm can be accurately controlled by measuring the thickness ofsubstance deposited in film by means of a film thickness monitor using aquartz oscillator. At the vapor deposition, the temperature of substratemay be equal to room temperature, or the substrate may be heated.

When the bonding member layer can be formed by the CVD method from agaseous material of, for example, silicon, amorphous carbon, carbonnitride, titanium, silicon carbide or W, it is appropriate to employ theCVD method. The formation of bonding member layer according to thechemical vapor deposition method can be appropriately performed by theuse of parallel plate plasma CVD apparatus. In this method, a raw gassuch as SiH₄ diluted with a diluent gas such as hydrogen gas accordingto necessity is introduced in a reaction vessel evacuated to vacuum.Radiofrequency power is applied to one of a pair of parallel plateelectrodes arranged opposite to each other, thereby generatingradiofrequency gas plasma. Thus, a film of silicon, amorphous carbon,carbon nitride, titanium, silicon carbide or W can be formed on asubstrate disposed on the other electrode opposite to the aboveelectrode. The substrate is generally heated to about 50 to 500° C.although varied depending on film growing conditions. Measuring of thefilm forming speed for each forming condition in advance would enableaccurate estimation of a film thickness by controlling a film formationtime. As the diluent gas, use can be made of nondeposited gas such ashelium, nitrogen, argon, xenon, neon or krypton as well as hydrogen gas.

The shape of the bonding member layer can be arbitrarily varied byetching after film formation or by masking the substrate at the time offilm formation.

In the production process of the present invention, the formation of thebonding member layer is not only important for preventing the peeling orcracking of the diamond film layer formed on the bonding member layerbut also extremely important for enhancing the product quality of thediamond film formed.

Therefore, in one preferred mode of the present invention, the bondingmember layer is constituted of a crystalline substance orientated for aspecified crystal face. The formation of the bonding member layer fromthe crystalline substance orientated for a specified crystal face wouldenable maintaining the orientation of the sublayer at the time offorming a polycrystalline diamond film thereon in accordance with thevapor phase method, thereby attaining growth of a polycrystallinediamond film of high orientation. As a result the diamond film of highcrystallinity can be obtained.

In another preferred mode of the present invention, the bonding memberlayer may be formed from a conductive substance. When the bonding memberlayer is formed from a conductive substance, the bonding member layer isused as an electrode, and a direct current voltage is applied to thebonding member layer at the time of forming the diamond film inaccordance with the vapor phase method. Consequently, an effectiveprecursor for diamond synthesis would be preferentially deposited on thebonding member layer, thereby enabling formation of a polycrystallinediamond film of large crystal grains with high crystallinity. Thecrystallinity increase and crystal grain enlargement of polycrystallinediamond film would enhance the thermal conductivity of the diamond film.As a result, a heat sink which is excellent in heat radiationcharacteristics can be obtained.

Although the method of forming the bonding member layer from thecrystalline substance orientated for a specified crystal face is notparticularly limited, when it is intended to form the bonding memberlayer whose principal component is silicon in accordance with the CVDmethod, the orientation of silicon film can be controlled by regulatingforming conditions. Thus, such a polycrystalline silicon film thatdiffraction peak assigned to crystal face (111), (220) or (400)preferentially appears when an X-ray diffractometry is conducted can beformed. For example, when it is intended to obtain a silicon filmpreferentially orientated on face (111) (when an X-ray diffractometry isconducted, the diffraction peak strength assigned to this face issignificantly greater than those assigned to other faces), filmformation is appropriately performed at high forming operationtemperatures. When it is intended to obtain a silicon filmpreferentially orientated on face (220) film formation is appropriatelyperformed under high reaction pressures. When it is intended to obtain asilicon film preferentially orientated on face (400), film formation isappropriately performed under such conditions that a silane halide gasand a silane hydride gas are mixed together at an appropriate ratio. Atthis stage, a mixture of reactive gas with a gasifiable compoundcontaining a dopant element selected from among the elements belongingto Group III or Group V of the periodic table, such as diborane orphosphine, can be subjected to film synthesis to thereby enablesynthesizing a conductive silicon film of P type or N type. When thebonding member layer is constituted of such a silicon film, both theabove orientation effect and voltage application effect can be exerted.Therefore, it is optimal to form, as the bonding member layer, apolycrystalline silicon film containing a dopant (namely, of P type or Ntype) which realizes preferential appearance of crystal face (111),(220) or (400) when an X-ray diffractometry is conducted.

In the production process of the present invention, the thus formedbonding member layer is overlaid with the diamond film. Asaforementioned, although the area, configuration, thickness, etc. offormed diamond film can appropriately be determined depending on themounted element, desired heat radiation characteristics, production timeand production cost, it is preferred that the thickness of the diamondfilm be in the range of 10 to 300 μm, especially 20 to 250 μm.

Although the method of forming the diamond film is not particularlylimited, it is preferred that film formation be effected in accordancewith the vapor phase method from the viewpoint that intended filmformation can be accomplished easily. As the vapor phase method, commonvapor phase methods suitable for diamond film formation, such aschemical vapor deposition and laser abrasion methods, can be employedwithout limitation. Of these, the chemical vapor deposition method ismost appropriate since, among the current technologies, it can providemeans for stably producing a highly crystalline diamond film with highreproducibility. The chemical vapor deposition method can be classifiedinto techniques respectively utilizing radiofrequency wave, microwave, ahot filament, etc., according to production process characteristics. Ofthese, the technique utilizing microwave (microwave CVD method) and thetechnique utilizing a hot filament (hot filament CVD method) arepreferred. These productive techniques will be described below.

In these productive techniques, carbonaceous gasifiable substancesincluding methane, acetylene, carbon dioxide and carbon monoxide aregenerally used as the raw material of diamond film. These depositedgases may be diluted with a nondeposited gas of, for example, hydrogen,oxygen, nitrogen, argon, xenon, neon or krypton.

Also, a gasifiable compound containing an element belonging to Group IIIor Group V of the periodic table, such as diborane or phosphine, can bemixed with the above gases before diamond synthesis. When a diamond filmsynthesis is carried out in the presence of such gases, a conductivediamond film of P type or N type can be synthesized.

The substrate temperature during the formation of diamond film, althoughnot particularly limited, is preferably in the range of 600 to 1200° C.,still preferably 700 to 1100° C. When the substrate temperature is lowerthan 600° C., a diamond film containing amorphous carbon in highproportion would be formed, so that the thermal conductivity would below, thereby disenabling satisfactory exertion of the effects of thepresent invention. On the other hand, when the substrate temperatureexceeds 1200° C., the bonding member layer may suffer damages. Further,amorphous carbon maybe contained in the diamond like the phenomenonoccurring in the low temperature film formation. Therefore, thesubstrate temperature exceeding 1200° C. is unfavorable. Any method ofheating the substrate can be employed without particular limitation aslong as the temperature can be set for the above ranges. For example,there can be mentioned a heating method wherein a heater is buried in aholder provided for mounting the substrate; a method wherein thesubstrate is heated by radiofrequency induction heating; and a methodwherein, in the microwave plasma CVD method, the substrate is heated bymicrowave inputted for plasma generation. The pressure for diamondsynthesis is generally in the range of 0.1 mTorr to 300 Torr. Inparticular, in the microwave plasma CVD method, the pressure is in therange of 50 mTorr to 200 Torr. Further, in the microwave plasma CVDmethod, the output of plasma generation power source, althoughappropriately selected depending on the characteristics of formeddiamond film, is generally in the range of 300 W to 10 kW. The shape ofdiamond film can be arbitrarily varied by etching after film formationor by masking the substrate at the time of film formation.

With respect to the process for producing the heat sink “A” of thepresent invention as shown in FIG. 1 wherein the bonding member layer isconstituted of silicon, particular modes thereof wherein use is made ofparallel plate type radiofrequency plasma CVD apparatus “B” as shown inFIG. 2 and also microwave plasma CVD apparatus “C” as shown in FIG. 3will be described in greater detail below.

The apparatus “B” of FIG. 2 is a representative apparatus which canappropriately be used in the formation of bonding member layer 110. Theapparatus “B” includes reaction vessel 201 constituted of, for example,a stainless steel such as SUS 304, wherein vacuum is maintained. Thereaction vessel 201 is connected through exhaust ports 203 a, 203 b,which are provided in a side wall of reaction chamber, to a vacuumsource such as a vacuum pump, so that given vacuum is maintained in thereaction vessel 201. In FIG. 2, numerals 205 and 207 a denote a turbomolecular pump and an oil rotary pump, respectively. High vacuum can beattained in the reaction vessel by exhausting by means of these pumps.Numeral 206 denotes a mechanical booster pump, and numeral 207 b denotesan oil rotary pump. These pumps are used at the time of synthesizing asilicon film. Further, vacuum valves 204 a, 204 b for regulating theexhaust rate are disposed. Still further, sample table 202 a formounting substrate 213 is disposed inside the reaction vessel 201 of theapparatus “B”. Heater 214 for heating the substrate 213 is buried inthis sample table so that there is provided means for enabling controlof the substrate temperature. This sample table is provided so as topass through a bottom wall of the reaction vessel 201 and is soconstructed as to be vertically slidable by drive means not shown,thereby permitting position adjustment. Slide portion, not shown,between the sample table 202 a and the bottom wall of reaction vessel201 is fitted with a sealing or seal member for ensuring desired vacuumlevel in the reaction vessel 201. Radiofrequency application electrode202 b is disposed opposite to the sample table 202 a for mounting thesubstrate, so that radiofrequency wave from radiofrequency oscillator212 can be led into the reaction vessel 201 via tuning unit 211.Further, reactive gas supply ports 208 a, 208 b are disposed at an upperportion of the reaction vessel 201, so that gases can be introduced intothe reaction vessel 201 via reactive gas flow rate regulator 209.Radiofrequency gas plasma is generated between the parallel plateelectrodes (202 a-202 b) of the reaction vessel 201 by simultaneouslycarrying out feeding of reactive gas and application of radiofrequencywave, so that the reactive gas can be dissociated to thereby form thesilicon film on the substrate 213.

The apparatus “C” of FIG. 3 is a representative apparatus which canappropriately be used in the formation of diamond film 120. Theapparatus “C” includes reaction vessel 301 constituted of, for example,a stainless steel such as SUS 304, wherein vacuum is maintained. Thereaction vessel 301 is connected through exhaust ports 303 a, 303 b,which are provided in a side wall of reaction chamber, to a vacuumsource such as a vacuum pump, so that given vacuum is maintained in thereaction vessel 301. In FIG. 3, numerals 305 and 307 a denote a turbomolecular pump and an oil rotary pump, respectively. High vacuum can beattained in the reaction vessel 301 by exhausting by means of thesepumps. Numeral 306 denotes a mechanical booster pump, and numeral 307 bdenotes an oil rotary pump. These pumps are used at the time ofsynthesizing a diamond film. Further, vacuum valves 304 a, 304 b forregulating the exhaust rate are disposed. Still further, sample table302 for mounting substrate 313 is disposed inside the reaction vessel301 of the apparatus “C”. Heater 314 for heating the substrate 313 isburied in this sample table so that there is provided means for enablingcontrol of the substrate temperature. This sample table is provided soas to pass through a bottom wall of the reaction vessel 301 and is soconstructed as to be vertically slidable by drive means not shown,thereby permitting position adjustment. Slide portion, not shown,between the sample table 302 and the bottom wall of reaction vessel 301is fitted with a sealing or seal member for ensuring desired vacuumlevel in the reaction vessel 301. Microwave transmission window 315constituted of a dielectric substance, such as quartz or alumina, isdisposed at an upper portion of the reaction vessel 301, so thatmicrowave from microwave oscillator 312 can be led through tuning unit311 and propagated through a microwave guide tube into the reactionvessel 301. Further, reactive gas supply ports 308 a, 308 b are disposedat an upper portion of the reaction vessel 301, so that gases can beintroduced into the reaction vessel 301 via reactive gas flow rateregulator 309. Microwave gas plasma is generated above the substratemounting table of the reaction vessel 301 by simultaneously carrying outfeeding of reactive gas and application of microwave, so that thereactive gas can be dissociated to thereby form the diamond film on thesubstrate 313. At this stage, the diamond film can be formed whileapplying a voltage to the silicon film as the bonding member layer.

When it is intended to produce the heat sink “A” of the presentinvention, first, substrate 213 is set on substrate mounting part 202 ainside the apparatus “B”. The interior of reaction vessel 201 isexhausted to create vacuum. After exhausting until a vacuum of 5×10⁻⁶Torr or below is realized in the reaction vessel 201, not only is gaswhose flow rate has been regulated by the reactive gas flow rateregulator fed through the reactive gas supply ports into the reactionvessel 201, but also radiofrequency wave from the radiofrequencyoscillator 212 provided outside the reaction vessel 201 is transmittedthrough tuner 211 to thereby minimize reflection loss and inputted tothe radiofrequency application electrode 202 b. Consequently,radiofrequency gas plasma is generated, so that a silicon film is formedon the substrate 213. The formation of silicon film is carried out whilethe pressure inside the reaction vessel 201 is preferably in the rangeof 0.1 mTorr to 100 Torr, still preferably 50 mTorr to 50 Torr. Underthis pressure, a uniform and homogeneous silicon film of highcrystallinity can be efficiently formed. In the production process ofthe present invention, the substrate temperature during the formation ofsilicon film, although not particularly limited, is preferably in therange of 50 to 500° C., still preferably 80 to 350° C. In theradiofrequency plasma CVD method, although the output of plasmageneration power source is appropriately selected depending on thecharacteristics of formed silicon film, it is generally in the range of5 W to 2 kW. The radiofrequency oscillation frequency is preferably inthe range of 1 to 200 MHz, still preferably 5 to 150 MHz. However, theseconditions depend on the capacity and morphology of apparatus used inthe above synthesis and should not be determined generally.

Gasifiable substances containing silicon, such as SiH₄, Si₂H₆, SiHCl₃,SiH₂Cl₂, SiCl₄, SiF₄ and SiF₂H₂, are generally used as the reactive gasfor deposition to be introduced in the reaction vessel 201. Thesedeposited gases may be diluted with a nondeposited gas of, for example,hydrogen, nitrogen, helium, argon, xenon, neon or krypton.

Also, a gasifiable compound containing an element belonging to Group IIIor Group V of the periodic table, such as diborane or phosphine, can bemixed with the above gases before the synthesis of silicon film. Whenthe synthesis of silicon film is carried out in the presence of suchgases, a conductive silicon film of P type or N type can be synthesized.

Although the introduction rates of reactive gas and diluent gas arevaried depending on production conditions, when it is intended to obtaina film of high crystallinity, it is generally preferred that the gasesbe introduced so that the total introduction rate falls within the rangeof 50 to 1000 cc/min. Further, although the ratio of reactive gas anddiluent gas mixed is not particularly limited, the greater the mixingratio of diluent gas to reactive gas (diluent gas/reactive gas), thegreater the tendency to formation of a silicon film of highcrystallinity. Also, the orientation of silicon film can be controlledby regulating the film formation temperature, reaction pressure and rawgas mixing ratio.

After the formation of the silicon film as the bonding member layer, theresultant substrate is taken out from the reaction vessel 201 and set onsubstrate mounting table 302 inside the apparatus “C” for diamond filmformation. In the same manner as in the formation of the bonding memberlayer, the interior of reaction vessel 301 is exhausted by a vacuum pumpto create vacuum. In the same manner as in the formation of the bondingmember layer, after exhausting until a pressure of 5×10⁻⁶ Torr or belowis realized in the reaction vessel 301, not only is gas whose flow ratehas been regulated by the reactive gas flow rate regulator fed throughthe reactive gas supply ports into the reaction vessel 301, but alsomicrowave from the microwave power source 312 provided outside thereaction vessel 301 is transmitted through tuner 311 to thereby minimizereflection loss, led through the microwave guide tube 310 and inputtedinto the reaction vessel 301. Consequently, microwave gas plasma isgenerated above the substrate mounting table 302, so that a diamond filmis formed on the substrate 313 having already been provided with thebonding member. At the stage of diamond film formation, application of adirect current voltage to the silicon film as the bonding member layerfor about an hour from the initial stage of film growth is especiallyeffective in the forming of a diamond film of high crystallinity. Theapplied voltage is preferably in the range of +500 V to −500 V. Theformation of diamond film is carried out while the pressure inside thereaction vessel 301 is preferably in the range of 0.1 mTorr to 300 Torr,still preferably 50 mTorr to 200 Torr. Under this pressure, a uniformand homogeneous diamond film of high crystallinity can be efficientlyformed. In the production process of the present invention, thesubstrate temperature during the formation of diamond film, although notparticularly limited, is preferably in the range of 600 to 1200° C.,still preferably 700 to 1100° C. In the microwave plasma CVD method,although the output of plasma generation power source is appropriatelyselected depending on the characteristics of formed diamond film, it isgenerally in the range of 300 W to 10 kW. The microwave oscillationfrequency is preferably in the range of 500 MHz to 5 GHz, stillpreferably 1 GHz to 4 GHz. However, these conditions depend on thecapacity and morphology of apparatus used in the above synthesis andshould not be determined generally.

Carbonaceous gasifiable substances such as methane, acetylene, carbondioxide and carbon monoxide are generally used as the reactive gas to beintroduced in the reaction vessel 301. These deposited gases may bediluted with a nondeposited gas of, for example, hydrogen, nitrogen,helium, argon, xenon, neon, krypton or oxygen.

Also, a gasifiable compound containing an element belonging to Group IIIor Group V of the periodic table, such as diborane or phosphine, can bemixed with the above gases before the synthesis of diamond film. Whenthe synthesis of diamond film is carried out in the presence of suchgases, a conductive diamond film of P type or N type can be synthesized.

Although the introduction rates of reactive gas and diluent gas arevaried depending on production conditions, when it is intended to obtaina diamond film of high thermal conductivity, it is generally preferredthat the gases be introduced so that the total introduction rate fallswithin the range of 50 to 6000 cc/min. Further, although the ratio ofreactive gas and diluent gas mixed is not particularly limited, thegreater the mixing ratio of diluent gas to reactive gas (diluentgas/reactive gas), the greater the tendency to formation of a diamondfilm of high crystallinity.

EXAMPLE

The present invention will be further illustrated below with referenceto the following Examples, which, however, in no way limit the scope ofthe invention.

In the following Examples and Comparative Examples, each silicon filmwas formed with the use of apparatus of the structure shown in FIG. 2,and each diamond film was formed with the use of apparatus of thestructure shown in FIG. 3. Moreover, in each of the following Examplesand Comparative Examples, the bonding member layer, diamond film andfinally obtained heat sink were evaluated according to the followingmethods (1) to (4).

(1) Measurement of Film Thickness

In the measuring of the thickness of bonding member layer, in advance, asilicon or tungsten film is formed on a quartz substrate, and the filmthickness is measured with the use of a tracer type film thicknessmeter. The measured thickness is divided by production time, therebydetermining the film forming rate beforehand. The thickness ofparticular bonding member layer was calculated by multiplying the filmforming rate by the time taken in the formation of the particularbonding member layer. On the other hand, the thickness of diamond filmwas determined by observing a section configuration through a scanningelectron microscope.

(2) Orientation

The orientation of silicon film and diamond film was inspected by anX-ray diffractometry. With respect to the silicon film, the peaksassigned to faces (111), (220), (311) and (400) appear at 28.4° (2θ/°),47.3° (2θ/°), 56.1° (2θ/°) and 69.1° (2θ/°), respectively. On the otherhand, with respect to the diamond film, the peaks assigned to faces(111), (220), (311) and (400) appear at 43.95° (2θ/°), 75.40° (2θ/°),91.60° (2θ/°) and 119.7° (2θ/°), respectively. Accordingly, theorientations were evaluated by comparing the intensities thereof witheach other.

(3) Crystallinity

The crystallinity was evaluated by measuring the half-value width ofscattered waveform appearing at about 1333 cm⁻¹ in Raman scatteringspectroscopy. The smaller the half-value width or Full Width HalfMaximum (FWHM), the higher the crystallinity.

(4) Thermal Conductivity

The thermal conductivity was calculated by the formula: $\begin{matrix}{{{thermal}\quad {conductivity}\quad \left( {W\text{/}m\quad K} \right)} = \quad {{density}\quad \left( {g\text{/}{cm}^{3}} \right) \times {specific}\quad {heat}\quad \left( {J\text{/}g\quad K} \right) \times}} \\{\quad {{thermal}\quad {diffusivity}\quad \left( {{cm}^{2}\text{/}s} \right) \times}} \\{\quad {100\quad {({constant}).}}}\end{matrix}$

Herein, the density was measured by the underwater density measuringmethod, and the thermal diffusivity was determined by the nonlinearregression analysis according to the 2-dimensional ring method.

Example 1

Ceramic substrate (25 mm×25 mm×0.5 mm thickness) containing aluminumnitride as a principal component was set on the substrate mounting tableinside the radio frequency plasma CVD apparatus. The interior of thereaction vessel was exhausted to vacuum, and simultaneously thesubstrate mounting table was heated to 120° C. The apparatus was heldundisturbed for about 30 min until the substrate temperature wasstabilized, and the reduction of pressure within the reaction vessel to5×10⁻⁶ Torr or below was confirmed. Monosilane gas and hydrogen wereintroduced in the reaction vessel at respective flow rates of 3 cc/minand 100 cc/min, and the internal pressure of the reaction vessel was setfor 1.5 Torr by regulating the exhaust valve. Subsequently,radiofrequency wave from the radiofrequency power source at an output of50 W was tuned by the tuner so as to minimize reflection loss andinputted to the radiofrequency application electrode. Radiofrequencypower was applied for about 2000 sec so that the silicon film of 100 nmthickness was deposited on the substrate. After the completion ofdeposition reaction, the residual gas inside the reaction vessel wasdrawn off, and, after the confirmation of the lowering of substratetemperature to 100° C. or below, the reaction vessel was opened toexpose the same to the atmosphere. Then, the substrate overlaid with thesilicon film was taken out from the radiofrequency plasma CVD apparatus.The orientation of the formed silicon film was inspected, and no resultof strong orientation on a specified face was recognized. Further, thethermal conductivity of the substrate having the silicon film depositedthereon was measured. The measurement showed that the thermalconductivity was about 200 W/mK.

Thereafter, in order to superimpose the diamond film, the obtainedsubstrate was set on the substrate mounting table inside the microwaveplasma CVD apparatus. The interior of the reaction vessel was exhaustedto vacuum, and simultaneously the substrate mounting table was heated to1000° C. The apparatus was held undisturbed for about 1 hr until thesubstrate temperature was stabilized, and the reduction of pressurewithin the reaction vessel to 5 ×10⁻⁶ Torr or below was confirmed.Methane gas and hydrogen were introduced in the reaction vessel atrespective flow rates of 12 cc/min and 300 cc/min, and the internalpressure of the reaction vessel was set for 100 Torr by regulating theexhaust valve. Subsequently, microwave from the microwave power sourceat an output of 5 kW was tuned by the tuner so as to minimize reflectionloss, led through the window constituted of quartz and introduced in thereaction vessel. Microwave power was applied for about 10 hr so that thediamond film of 50 μm thickness was deposited on the substrate. Afterthe completion of deposition reaction, the lowering of substratetemperature to 100° C. or below was confirmed, and thereafter thereaction vessel was opened to expose the same to the atmosphere. Then,the substrate overlaid with the silicon film and the diamond film wastaken out from the microwave plasma CVD apparatus. The diamond film onthe substrate was visually inspected, and it was found that formation ofthe diamond film extended to edges of the substrate and that there wasno film peeling. A section of the diamond film was observed through amicroscope, and the thickness of the diamond film was measured. It wasfound that the thickness was about 50 μm. Thereafter, the crystallinityof the diamond on the substrate was estimated by Raman scatteringspectroscopy. As a result, it was found that the FWHM of peak assignedto diamond structure, appearing at about 1333 cm⁻¹, was about 8.5 cm⁻¹.Further, the orientation of the diamond layer was inspected, and, asfound with respect to the bonding member layer, no result of strongorientation on a specified face was recognized. Still further, thethermal conductivity of the obtained laminate (heat sink) was measured.The measurement showed that the thermal conductivity was about 350 W/mK.Calculation from the thermal conductivity values showed that the thermalconductivity of the diamond film was 1850 W/mK.

Example 2

Heat sink was produced in the same manner as in Example 1 except thatthe thickness of synthesized diamond film was 200 μm. The diamond filmon the substrate was visually inspected, and it was found that formationof the diamond film extended to edges of the substrate and that therewas no film peeling. A section of the diamond film was observed througha microscope, and the thickness of the diamond film was measured. It wasfound that the thickness was about 200 μm. Thereafter, the crystallinityof the diamond on the substrate was inspected in the same manner as inExample 1. As a result, it was found that the FWHM was about 6.8 cm⁻¹.Further, the orientation of the obtained diamond layer was inspected,and no result of strong orientation on a specified face was recognized.Still further, the thermal conductivity of the obtained heat sink wasmeasured. The measurement showed that the thermal conductivity afterlamination with the diamond film was about 720 W/mK while that afterlamination with the bonding member layer was about 200 W/mK. Calculationfrom these values showed that the thermal conductivity of the diamondfilm was 2000 W/mK.

Example 3

Heat sink was produced in the same manner as in Example 1 except thatthe bonding member layer was constituted of W. The diamond film on thesubstrate was visually inspected, and it was found that formation of thediamond film extended to edges of the substrate and that there was nofilm peeling. A section of the diamond film was observed through amicroscope, and the thickness of the diamond film was measured. It wasfound that the thickness was about 50 μm. Thereafter, the crystallinityof the diamond on the substrate was inspected. As a result, it was foundthat the FWHM was about 8.0 cm⁻¹. Further, the orientation of theobtained diamond layer was inspected, and no result of strongorientation on a specified face was recognized. Still further, thethermal conductivity of the obtained heat sink was measured. Themeasurement showed that-the thermal conductivity after lamination withthe diamond film was about 360 W/mK while that after lamination with thebonding member layer was about 210 W/mK. Calculation from these valuesshowed that the thermal conductivity of the diamond film was 1860 W/mK.

Comparative Example 1

Heat sink was produced in the same manner as in Example 1 except that nobonding member layer was formed. The diamond film on the substrate wasvisually inspected, and film peeling was found in the vicinity ofsubstrate edges.

Comparative Example 2

The same procedure for producing a heat sink as in Example 3 wasrepeated except that the material of the bonding member layer waschanged to nickel. However, a uniform diamond film of desired thicknesscould not be obtained. This showed that nickel was unsuitable to thebonding member.

Example 4

Silicon film of 100 nm thickness was deposited on the substrate in thesame manner as in Example 1 except that the substrate mounting table washeated to 300° C. and that the output of radiofrequency power source was10 W. The orientation of obtained silicon film was inspected in the samemanner as in Example 1. It was found that the silicon film hadorientation of (111). Further, the thermal conductivity of the substratehaving the silicon film deposited thereon was inspected. It was foundthat the thermal conductivity was about 200 W/mK.

Thereafter, a diamond film of 50 μm thickness was formed on the abovesubstrate having the silicon film deposited thereon in the same manneras in Example 1. Thus, a heat sink was obtained. The crystallinity ofthe diamond on the substrate was inspected. As a result, it was foundthat the FWHM was about 7.9 cm⁻¹. Further, the orientation of theobtained diamond layer was inspected, and realization of such a resultthat diffraction peak assigned to face (111) was larger than the otherpeaks by about 2.5 times was recognized. Still further, the thermalconductivity of the obtained heat sink was measured. The thermalconductivity was about 370 W/mK. Calculation from the thermalconductivity values showed that the thermal conductivity of the diamondfilm was 2070 W/mK.

Example 5

Heat sink was produced in the same manner as in Example 4 except that,with respect to the conditions for forming the silicon film, thesubstrate was heated to 120° C. Various evaluations of the heat sinkwere effected. As a result, it was found that, with respect to thecrystallinity of the diamond on the substrate, the FWHM was about 7.7cm⁻¹. Further, with respect to both the silicon film and the diamondfilm, realization of such a result that diffraction peak assigned toface (220) was larger than the other peaks by about 4 times wasrecognized. Still further, the thermal conductivity values of thesubstrate having the silicon film deposited thereon and the finallyobtained heat sink were 200 W/mK and 380 W/mK, respectively. Calculationfrom these thermal conductivity values showed that the thermalconductivity of the diamond film was 2180 W/mK.

Example 6

Heat sink was produced under the same conditions as in Example 5 exceptthat diborane was fed at a flow rate of 5 cc/min at the stage of formingthe bonding member layer and that a direct current voltage of −100 V wasapplied to the bonding member layer for 30 min in the initial stage ofdiamond film formation. Various evaluations of the heat sink wereeffected. As a result, it was found that, with respect to thecrystallinity of the diamond on the substrate, the FWHM was about 7.3cm⁻¹. Further, with respect to both the silicon film and the diamondfilm, realization of such a result that diffraction peak assigned toface (220) was larger than the other peaks by about 4 times wasrecognized. Still further, the thermal conductivity values of thesubstrate having the silicon film deposited thereon and the finallyobtained heat sink were 200 W/mK and 400 W/mK, respectively. Calculationfrom these thermal conductivity values showed that the thermalconductivity of the diamond film was 2400 W/mK.

The heat sink of the present invention exhibits high heat radiationefficiency by virtue of the use of a ceramic substrate constitutedmainly of aluminum nitride of high thermal conductivity (AlN substrate)as a substrate on which a diamond film is formed. Further, the substrateis an insulator, so that it is practicable to cover part of thesubstrate with a diamond film while a circuit is formed on the otherpart of the substrate. Still further, in the heat sink of the presentinvention, the substrate and the diamond film are bonded together withsatisfactory strength, so that, even if a heating/cooling heat cycle isrepeated during the use thereof, the diamond film would not sufferpeeling or cracking. Therefore, the heat sink can be stably used for aprolonged period of time.

Moreover, the production process of the present invention enablesefficiently producing the above heat sink of the present invention.Further, whilst the prior art of forming a diamond film directly on theAlN substrate in accordance with the vapor phase method has failed toform a diamond film of high quality, the production process of thepresent invention enables forming a diamond film of high quality on theAlN substrate, although indirectly, by virtue of the interposition of abonding member layer of specified substance.

What is claimed is:
 1. A heat sink comprising a ceramic substrate havingat least one plane containing aluminum nitride as a principal component,and, a diamond film layer superimposed on the plane of the ceramicsubstrate, characterized in that the ceramic substrate and the diamondfilm layer are bonded together via a bonding member which is constitutedof a crystalline substance orientated for a specified crystal faceinterposed therebetween.
 2. The heat sink as claimed in claim 1, whereinthe bonding member is constituted of at least one material selected fromthe group consisting of silicon, silicon carbide, tungsten, tungstencarbide, CuW, Cu—Mo alloy, Cu—Mo—W alloy, amorphous carbon, boronnitride, carbon nitride and titanium.
 3. The heat sink as claimed inclaim 1, wherein the bonding member is constituted of a polycrystallinesilicon orientated for crystal face (111), crystal face (220) or crystalface (400).
 4. The heat sink as claimed in claim 3, wherein thepolycrystalline silicon contains a dopant.
 5. The heat sink as claimedin claim 1, wherein the diamond film layer exhibits a thermalconductivity of 800 W/mK or greater.
 6. A process for producing a heatsink, comprising the steps of: providing a ceramic substrate having atleast one plane containing aluminum nitride as a principal component,forming a bonding member layer which is constituted of a crystallinesubstance orientated for a specified crystal face on at least a portionof the plane of the ceramic substrate, and forming a diamond film on thebonding member layer.
 7. The process as claimed in claim 6, wherein thebonding member layer is formed from a conductive substance, and, while adirect current voltage is applied to the bonding member layer, a thindiamond film is formed on the bonding member layer in accordance withmicrowave CVD or hot filament CVD method.
 8. The heat sink as claimed inclaim 2, wherein the diamond film layer exhibits a thermal conductivityof 800 W/mK or greater.
 9. The heat sink as claimed in claim 3, whereinthe diamond film layer exhibits a thermal conductivity of 800 W/mK orgreater.
 10. The heat sink as claimed in claim 4, wherein the diamondfilm layer exhibits a thermal conductivity of 800 W/mK or greater.